The abuse of phencyclidine (PCP) and related arylcyclohexylamine derivatives has become a widespread medical and social problem. It is now widely recognized that the abuse potential of a drug is closely related to the qualitative nature and intensity of the subjective effects that it can engender in the user. Psychoactive drugs can also serve as discriminative stimuli for controlling behavior in animals in much the same manner as traditional exteroceptive stimuli. An increasing body of literature suggests that the properties of a drug which enable it to function as a discriminative stimulus in animals are analogous to the component of action responsible for producing subjective effects in man. Thus, a comprehensive study of the discriminative stimulus properties of PCP and related compounds in animals should provide systematically-derived quantitative data on a component of drug action that underlies abuse potential in man. Accordingly, the objective of the proposed research is to develop and validate procedures in the rat and squirrel monkey for studying the stimulus properties of PCP. Animals will be trained to discriminate between the administration of saline and PCP in a two-choice discrete trial avoidance paradigm. Specific experiments will be aimed at determining: 1) similarities and differences between the stimulus properties of PCP and those of other prototypic psychoactive drugs; 2) whether or not tolerance develops to the stimulus effects of PCP; 3) structure-activity relationships for producing PCP-like stimulus effects; and, 4) neurochemical and neuroanatomical substrates that mediate stimulus control of behavior by PCP.